PROJECT SUMMARY/ABSTRACT Through the development and maintenance over the past 13 years, CHARMM-GUI (http://www.charmm-gui.org) has become an essential web-based resource for the interactive construction of complex biomolecular simulation systems and the preparation of their inputs with well-established and reproducible simulation protocols for state- of-the-art molecular simulations using widely used programs, such as CHARMM, NAMD, GROMACS, AMBER, GENESIS, TINKER, LAMMPS, Desmond, and OpenMM. CHARMM-GUI has also significantly reduced the learning curve that prevents non-experts (e.g., experimentalists) from utilizing modeling and simulation to study their systems. In this application, following requests from CHARMM-GUI users and broad MD simulation package developers, three specific aims are proposed to extend the functionality of CHARMM-GUI to support a broader range of modeling and simulation community with more diverse force field selection and advanced techniques. Specifically, AIM1 is to support a more complete list of additive and polarizable force fields for all- and united-atom simulations. AIM 2 is to develop QM/MM Interfacer for QM and QM/MM calculations. Finally, AIM 3 will extend the functionality of CHARMM-GUI with non-CHARMM implicit solvent models, SILCS-based simulations (Site Identification by Ligand Competitive Saturation), ligand binding free energy calculations, constant pH simulations, replica-exchange simulations, ion transport simulations, targeted/steered simulations, and path sampling. The successful completion of the proposed project is expected to provide an effective one- stop online resource for biomedical research community to carry out innovative and novel biomolecular modeling and simulation research for the prevention and treatment of human disease.